Delivery mechanism, fixing device and image forming apparatus

ABSTRACT

A sheet delivery mechanism including: a first roll; a second roll; and a plurality of convexes formed circumferentially for pushing a rear end of a sheet to be delivered, wherein: a sheet delivery roll pair comprises the first roll and the second roll in contact with each other; at least one of the first roll and the second roll is provided with the plurality of convexes; and one of the plurality of convexes is arranged to be more inside radially than the tangential line drawn from the radial tip of another convex upstream adjacent in the rotating direction to the one convex at issue to the outer periphery of the roll.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sheet delivery mechanism in an image formingapparatus such as a printer, copier or facsimile and a fixing device andthe image forming apparatus having the sheet discharging mechanism.

2. Background Art

In the image forming apparatus, it is known that when a sheet with animage formed by an image forming section is delivered or discharged intoan output tray, a paper delivery roll pair consisting of a follower rolland pinch roll which sandwich a nip are used.

Until now, a sheet delivery mechanism is known which is a sheet deliveryroll serving as an inverting roll for both-side printing, provided witha slope on the side acting when the convex portion of the kicking shapeof a pinch roll is inverted (JP-B-7-17295). Another paper deliverymechanism is known in which the rising angle of the convex of thekicking shape on the side acting in delivery is set at 45° or less(JP-A-11-79514). Still another paper delivery has been disclosed inwhich an elastic blade-like convex is provided so that in sheet feedingthe convex is deformed inward in a radial direction and in kicking-out,the convex is acted on the sheet (JP-A-08-246034).

However, if the sheet is discharged with the rear end slanted downwardin a certain degree according to the convex in the kicking shape, thesheet interferes with the convex on the downstream side in a rotatingdirection and floats. Thus, the rear end of the sheet cannot be pushedby the portion of the kicking shape. This led to a problem of pooraccommodation due to the leaving the rear end of the sheet.

Further, in order to prevent damage when a thick sheet passes through anip and poor rotation, it is necessary to reduce the height of theconvexes attached to the pinch roll. This requires the performance ofkicking out the sheet rear end to be lowered. When the sheet climbs overthe convex, this influences the roll nip to generate bending of thesheet. Further, in the case of the elastic blade-like convex, if afixing roll is arranged in the vicinity of the delivery roll,time-passage deformation was inevitable which is attributable todegradation of the material due to heat or the like.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a sheet deliverymechanism which when a sheet is kicked out by a convex attached to atleast one of a first roll and second roll of a sheet delivery roll pair,can prevent poor accommodation due to leaving of a sheet rear endwithout being hindered by another convex located on the downstream sideof the convex.

A second object of this invention is to provide a sheet deliverymechanism which when can deliver a thick sheet without damaging it.

In order to attain the above object, the first feature of this inventionis to provide a sheet delivery mechanism characterized by including asheet delivery roll pair consisting of a first roll and a second roll incontact with each other, wherein at least one of the first roll and thesecond roll is provided with a plurality of convexes formedcircumferentially for pushing the rear end of a sheet to be delivered,and one of these plurality of convexes is arranged to be more insideradially than the tangential line drawn from the radial tip of anotherconvex upstream adjacent in the rotating direction to the one convex tothe outer periphery of the roll. In accordance with this configuration,even where the sheet rear end delivered may be left on the first roll orthe second roll, when the sheet rear end is kicked out by the conveximmediately passed a nip, it is not hindered by the convex located onthe downstream side of the above convex. This prevents pooraccommodation due to the leaving of the sheet rear end.

This invention is preferably a sheet delivery mechanism wherein aportion more upstream than the radial tip of the one convex has atapered shape with a gradient approximately equal to the tangential linedrawn from the radial tip of another convex upstream adjacent in therotating direction to the one convex to the outer periphery of the roll.In accordance with this configuration, as compared with the case where aportion more upstream than the radial tip of the convex does not have atapered shape with a gradient approximately equal to the tangential linedrawn from the radial tip of another convex upstream adjacent in therotating direction to the one convex to the outer periphery of the roll,a larger number of convexes can be arranged on the circumference of theroll and the number of times of kicking out the sheet rear end can beincreased. This prevents poor accommodation due to the leaving of thesheet rear end.

This invention is preferably a sheet delivery mechanism wherein thedistance from the root of the one convex to a rotation center is shorterthan the outer diameter of the roll.

In accordance with this configuration, even where the degree of theconvex from the outer periphery of the roll body is relatively small,the sheet rear end can be effectively pushed out. This obviates damageson the image on the sheet which is attributable to the convexes.

The second feature of this invention is to provide a sheet deliverymechanism characterized by including a a sheet delivery roll pairconsisting of a first roll and a second roll in contact with each other,wherein at least one of the first roll and the second roll is provided,on its roll body, with a plurality of convexes formed circumferentiallyfor pushing the rear end of a sheet to be delivered to shift inwardly inthe radial direction through their elastic deformation; and where asheet, which receives pressure not lower than a predetermined pressurewhen it passes a nip, passes the nip, the convex passing the sameposition as the nip in the rotating direction shifts inwardly in theradial direction, and where the sheet, which receives the pressure lowerthan the predetermined pressure when it passes the nip, passes the nip,the convex keeps its normal convex shape. In accordance with thisconfiguration, when the thick sheet which receives the pressure notlower than the predetermined pressure passes the nip, the thick sheetcan be prevented from being damaged owing to the convex shape.Conventionally, if the degree of the convex from the. outer periphery ofthe roll is increased, the convex must have a lower height permittingthe sheet to climb over the convex so that the performance of the convexof kicking out the sheet rear end was limited. But in the aboveconfiguration, the degree of the convex can be increased, theperformance of kicking out the sheet rear end can be improved.

This invention is preferably is a sheet delivery mechanism wherein theroll body and the convexes are integrally formed of resin, and theconvexes are formed in a lightening shape or a shape with recesses onboth ends in the axial direction. In accordance with this configuration,as compared with the conventional roll shape, without increasing theproduction cost and with no time-passage deterioration such as abrasionor settling of the convex, the performance of kicking-out the sheet rearend can be maintained and the image quality can be improved.

This invention is preferably a sheet delivery mechanism wherein the rollbody and the convexes are formed separately from each other and made ofdifferent materials from each other. In accordance with thisconfiguration, even where the sheet passing distance from the fixingdevice to the delivery roll pair is so short that the delivery roll isrequired to have the releasiablity, the image quality and performance ofkicking out the sheet rear end can be improved.

This invention is preferably a sheet delivery mechanism wherein the rollbody is made of the material selected from the group consisting oftetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA),polytetrafluoroethylene (PTFE) and tetrafluoroethylene-ethylenecopolymer (ETFE). For this reason, the roll made of the above materialcan be given improved slidability and toner releasiablity.

This invention is preferably a sheet delivery mechanism wherein theconvexes are made of the material selected from the group consisting ofpolyacetal (POM), polypropylene (PP) and polycarbonate (PC). For thisreason, the convexes can be given improved elastic force and abrasionresistance.

This invention is preferably a sheet delivery mechanism wherein thesheet delivery roll pair serves as an inverting roll for duplexprinting. In accordance with this configuration, in the case of duplexprinting, in the sheet delivery mechanism, the sheet can be fed out tothe inverting path.

This invention is preferably the fixing device wherein the sheetdelivery mechanism is internally arranged. In accordance with thisconfiguration, where the sheet delivery mechanism has broken down, thefixing device can be taken out from the image forming apparatus for itsreplacement or repair. For this reason, as compared with the repair orreplacement of the image forming apparatus itself, the cost for repairor replacement can be reduced.

This invention includes an image forming apparatus provided with thesheet delivery mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of this invention will becomemore fully apparent from the following detailed description taken withthe accompanying drawings in which:

FIG. 1 is a sectional view of an image forming apparatus according toembodiments of this invention;

FIG. 2 is a side view of the outline of a sheet delivery roll pairaccording to the first embodiment of this invention;

FIG. 3 is a side view of the outline of a sheet delivery roll pairaccording to the first embodiment of this invention;

FIG. 4 is an enlarged side view of the main part of a pinch roll of thedelivery roll pair shown in FIG. 2;

FIG. 5 is a side view of the outline of a sheet delivery roll pairaccording to the second embodiment of this invention;

FIG. 6 is a side view of the outline of a sheet delivery roll pairaccording to the second embodiment of this invention;

FIG. 7 is a side view of the outline of a sheet delivery roll pairaccording to the third embodiment of this invention;

FIG. 8 is a partial sectional view of the outline of a sheet deliveryroll pair according to the third embodiment of this invention;

FIG. 9 is a side view of the outline of a sheet delivery roll pairaccording to the fourth embodiment of this invention; and

FIG. 10 is a partial sectional view of the outline of a sheet deliveryroll pair according to the fourth embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawings, an explanation will be given of variousembodiments of this invention.

FIG. 1 is an outline of an image forming apparatus 10 according to anembodiment of this invention.

The image forming apparatus 10 has an image forming apparatus body 12and an opening/closing door 16 which is rotatable around a rotatingfulcrum 14 located at an upper position of the image forming apparatusbody 12.

At a lower position of the image forming apparatus body 12, e.g.single-stage recording medium feeding unit 18 is located. The recordingmedium feeding unit 18 includes a recording medium feeding unit body 20and a recording medium feeding cassette 22 in which recording media arehoused. At the upper positions of the depth end of the recording mediumfeeding cassette 22, located are a feed roll 24 for feeding recordingmedia from the recording medium feeding cassette 22 and a retard roll 26for individually dressing the recording media to be fed.

A transporting path 28 is a recording medium path from the feed roll 24to a delivery outlet 30 which is formed from the rear side of the imageforming apparatus body 12 toward the inside of the image formingapparatus body 12. The transporting path 28 is formed nearlyperpendicularly from the recording medium feeding cassette 22 to afixing device 90 described later. On the upper stream side of the fixingdevice 90 on the transporting path 28, arranged are a secondary transferroll 81 and a secondary backup roll 72. On the upstream side of thesecondary transfer roll 81 and secondary backup roll 72, arranged areresist rolls 32. Further, in the vicinity of the delivery outlet 30 ofthe transporting path 28, a delivery roll pair 34 is arranged which islocated as a lower pinch roll (first roll) 100 and an upper followerroll (second roll) 102 which are adapted to be in contact with eachother to sandwich a nip 35 therebetween, these rolls being arranged atupper and lower positions within the fixing device 90 as describedlater. The delivery roll pair 34 is adapted to be rotated in forward andreverse directions by a driving motor not shown.

Therefore, the recording media fed out from the recording medium feedingcassette 22 of the recording medium feeding unit 18 by the feed roll 24are dressed by the retard roll 26 so that only the uppermost recordingmedium thereof is guided onto the transporting path 28. The recordingmedium guided is temporarily stopped by the resist rolls 32, and at anappropriate timing through between the secondary transfer roll 81 andsecondary transfer roll 72, a developer is transferred to the recordingmedium. The image of the developer transferred is fixed by the fixingdevice 90. In the case of duplex printing, the recording medium guidedto the delivery roll pair 34, when the delivery roll pair 34 isinverted, is transported toward the resist rolls 32 again through aninverting path 95. Further, through between the secondary transfer roll81 and the secondary backup roll 72, a toner image is transferred to therecording medium, and fixed by the fixing device 90. The recoding mediumis delivered or discharged by the forward rotation of the delivery rollpair 34. A delivery tray 36 slopes so that it is low at the deliveryoutlet and becomes high toward the front (left direction in FIG. 1).

In the image forming apparatus body 12, for example, at the virtualcenter on the front side, a rotary developing device 38 is arranged. Therotary developing device 38 has four developing units 42 a to 42 d forforming four developer images of Yellow, Magenta, Cyan and Black,respectively within a developing device body 40. These developing units42 a to 42 d rotate counterclockwise in FIG. 1 around the center 44 ofthe rotary developing device 38. These developing devices 42 a to 42 dhave developing rolls 46 a to 46 d, respectively which are urged in anormal direction of the developing device body 40 by elastic bodies 48 ato 48 d such as coil springs. In other words, the developing rolls 46 ato 46 d of the developing devices 42 a to 42 d are arranged on the outerperiphery of the developing device body 40 at intervals of 90° aroundthe rotary developing device center 44, and in their contact with animage carrier 50 described later, they will visualize the electrostaticlatent image on the image carrier 50 by the developers of thecorresponding colors.

The image carrier 50 is arranged in contact with the rotary developingdevice 38. The image carrier 50 is made of a tubular aluminum pipecoated with a photoconductive layer on the surface. The photoconductivelayer of the image carrier 50 carries the electrostatic latent imageformed by a light beam such as a laser beam and developer imagevisualized by the developer.

Beneath the image carrier 50, a charging device 52 for uniformly theimage carrier 50 is arranged which is e.g. a charging roll. Further, incontact with the image carrier 50, a cleaner 54 for the image carrier isarranged on the more upstream side than the charging device 52 in therotating direction of the image carrier 50. The cleaner 54 for the imagecarrier includes a cleaning blade for scraping off the developerremaining on the image carrier 50 after e.g. primary transfer and adeveloper recovering bottle 58 for recovering the developer scraped offby the cleaning blade 56.

Incidentally, the developer recovering bottle 58 is provided, on therear side (right side in FIG. 1), with e.g. a rib which is curved sothat the recording medium is smoothly transported, thereby constitutinga part of the transporting path 28.

Below the rotary developing unit 38, an exposure device 60 is arrangedfor writing the latent image on the image carrier 50 charged by thecharging device 52 using the light beam such as a laser beam. Above therotary developing unit 38, an intermediate transferring device 62 isarranged for primary-transferring the developer image visualized by therotary developing unit 38 at a primary transfer position andtransporting the developer image transported to a secondary transferposition.

The intermediate transferring device 62 includes an intermediatetransfer body 64 such as an intermediate transfer belt 64, a primarytransfer roll 66, a lap-in-roll 68, a lap-out-roll 70, a secondarytransfer backup roll 72, a scraper backup roll 74 and brush backup roll76. The intermediate transfer body 64 has e.g. elasticity and isstretched nearly flatly to have a long side and short side above therotary developing unit 38. The long side on the upper side of theintermediate transfer body 64 is stretched so that it is in parallel tothe delivery tray 36 arranged at the upper position of e.g. the imageforming apparatus body 12. The intermediate transfer body 64 has aprimary transfer portion (image carrier lapping region). Theintermediate transfer body 64 has a primary transfer portion in contactwith the image carrier 50 in its lapping manner, on the lower side ofthe long sides of the intermediate transfer body 64, between thelap-in-roll 68 arranged upstream of the primary transfer roll 66 and thelap-out-roll 70 arranged downstream of the primary transfer roll 66.With the primary transfer portion lapping in its predetermined range,over the image carrier, the intermediate transfer body 64 follows therotation of the image carrier 50.

In this way, on the intermediate transfer body 64, the developer imageon the image carrier 50 is transferred primarily superposedly by theprimary transfer roll 66 in the order of e.g. yellow, magenta, cyan andblack. The developer image primarily transferred is transported to thesecondary transfer roll 81.

Incidentally, the lap-in-roll 68 and the lap-out-roll 70 are apart fromthe image carrier 50.

Further, on the rear side of the intermediate transfer body 64 (rightside in FIG. 1), the lap-in-roll 68 and the lap-out-roll 70 constitute aplanar section (short side) which serves as a secondary transfer portionfacing the transporting path 28.

Incidentally, in the secondary transfer body, the lap-out-roll 70 isarranged to form an angle of e.g. 12° between the intermediate transferbody 64 and the transporting path 28.

The scraper backup roll 74 assists a scraper 84 (described later) toscrape off the developer remaining on the intermediate transfer body 64after the secondary transfer, and the brush backup roll 76 assists abrush roll 86 (described later) to scrape off the developer remaining onthe intermediate transfer body 64 after the secondary transfer.

Above the long side of the intermediate transfer body 64, a sensor 79such as a reflecting type photo-sensor fixed to the rear side (inside)of the opening/closing door 16 is provided. The sensor 79 reads thepatch of the developer formed on the intermediate transfer body 64 todetect the position of the intermediate transfer body 64 in the rotatingdirection and detect the density of the developer.

The secondary transfer backup roll 72 of the intermediate transferringdevice 62 is opposite to the second transfer roll 81 across thetransporting path 28. Namely, the secondary transfer position in thesecondary transfer section is located between the secondary transferroll 81 and the secondary transfer backup roll 72. At the secondarytransfer position, the secondary transfer roll 81, under the assistanceof the secondary transfer backup roll 72, secondary-transfers thedeveloper image primary-transferred on the intermediate transfer body 64onto the recording medium. Now, the secondary transfer roll 81 isdesigned to be apart from the intermediate transfer body 64 while theintermediate transfer body 64 rotates three times, i.e. transports threedeveloper images of yellow, magenta and cyan, and to bring into contactwith the intermediate transfer body 64 when the developer image of blackis transferred.

Incidentally, a prescribed potential difference is generated between thesecondary transfer roll 81 and the secondary transfer backup roll 72.For example, if the secondary transfer roll 81 is at a high voltage, thesecondary transfer backup roll 72 is connected to ground (GND). Namely,the intermediate transfer body 64 is placed at a prescribed potentialbetween the secondary transfer roll 81 and secondary transfer backuproll 72.

A cleaner 83 for the intermediate transfer body 64 is arranged incontact with the side end of the intermediate transfer body 64 oppositeto the image carrier. The cleaner 83 for the intermediate transfer body64 includes a scraper 84 for scraping off to clean the developerremaining on the intermediate transfer body 64 after e.g. the secondarytransfer, a brush roll 86 for further scraping off the developerremaining after cleaning by the scraper 84 and a developer recoverybottle 88 for recovering the developer scraped off by the scarper 84 andbrush roll 86. The scraper 84 is made of e.g. a stainless steel plate. Avoltage having an opposite polarity to the developer is applied to thescraper 84. The brush roll 86 is formed of e.g. an acryl brush treatedto have conductivity. While the intermediate transfer body 64 istransporting the developer image, the scraper 84 and the brush roll 86are apart from the intermediate transfer body 64. At a predeterminedtiming, these components are integrated to bring into contact with theintermediate transfer body 64.

Above the secondary transfer position, the fixing device 90 is arranged.The fixing device 90 includes a heating roll 92 and a pressurizing roll94 to fix the developer image secondary-transferred on the recordingmedium by the secondary transfer roll 81 and the secondary transferbackup roll 72 onto the recording medium, and transports the recordingmedium toward the delivery roll pair 34 provided within the fixingdevice 90. Further, at an upper position of the fixing device 90, arecording medium passing opening (not shown) is provided. In the case ofduplex printing, the delivery roll pair 34 is inverted so that therecording medium is guided to the inverting path 95 and transportedtoward the. resist rolls 32 by means of transporting rolls 96 a to 96 c.

An image forming unit 98 is an integrated body consisting of theintermediate transferring device 62, image carrier 50, charging device52, cleaner 54 for the image carrier and cleaner 83 for the intermediatetransfer body. This image forming unit 98 is arranged immediately belowthe delivery tray 36. The image forming unit is made detachable from theimage forming device body 12 by opening the opening/closing door 16.

Next, referring to the drawings, an explanation will be given of thedelivery roll pair 34 according to the first embodiment of thisinvention.

FIG. 2 is a side view of the status in the vicinity of the delivery rollpair 34 according to the first embodiment of this invention. FIG. 3 is aside view of the delivery roll pair 34 shown in FIG. 2 when it is viewedfrom the side of the delivery tray 36.

As seen from FIGS. 2 and 3, the delivery roll pair 34 consists of alower pinch roll (first roll) 100 and an upper follower roll (secondroll) 102 which sandwich the nip 35 therebetween. The pinch roll 100 isprovided in a status wider than the follower roll 102 in an axialdirection, and located on the side nearer the delivery tray 36 than thefollower roll 102.

At the center of rotation of the pinch roll 100, a rotary axis 103 forrotation-driving the pinch roll 100 is provided. The pinch roll 100includes a roll body 104 and a plurality of convexes 106 formedcircumferentially on both ends of the roll body 104. The convexes 106are provided for pushing the rear end of the sheet to be delivered ordischarged. Assuming, as a standard circle, a circle constituted by theouter periphery of the roll body 104 when it is cut in a sectionperpendicular to the axial direction of the roll body 104, each of theconvexes 106 protrudes radially from a standard circle.

Incidentally, because the pinch roll 100 is provided to be wider thanthe follower roll 102, these convexes 106 will not bring into contactwith the follower roll 102.

One of these convexes 106, as shown in FIG. 4 which is an enlarged viewof the main part of FIG. 2, is arranged to be more inside radially thanthe tangential line drawn from the radial tip 107 of another convex 106upstream adjacent in the rotating direction to the convex 106 at issueto the outer periphery of the roll body 104 of the pinch roll 100.

Thus, even where the rear end of the sheet delivered by the deliveryroll pair 34 may be left on the pinch roll 100, without being hinderedby the convex 106 after passed the nip 35, the rear end of the sheet canbe kicked out by another convex 106 upstream in the rotating direction.This prevents poor accommodation due to the leaving of the sheet rearend.

Incidentally, the portion of the convex 107 of the convex 106 moredownstream in the rotating direction than the radial tip 107 of theconvex 106 is formed to be nearly perpendicular to the outer peripheryof the roll body 104 of the pinch roll 100, which is a shape preferableto kick out the sheet rear end.

On the other hand, the portion of the convex 107 more upstream in therotating direction than the radial tip 107 of the convex 106 has atapered shape with a gradient approximately equal to the tangential linedrawn from the radial tip 107 of another convex 106 upstream adjacent inthe rotating direction to the convex 106 at issue to the outer peripheryof the roll body 104 of the pinch roll 100. Thus, in this way, ascompared with the case the portion of the convex 107 more upstream inthe rotating direction than the radial tip 107 of the convex 106 doesnot have a gradient equal to the tangential line drawn from the radialtip 107 of another convex 106 upstream adjacent in the rotatingdirection to the convex 106 at issue to the circumference of the rollbody 104 of the pinch roll 100, a larger number of convexes can bearranged on the outer periphery of the pinch roll 100 and the number oftimes of kicking out the sheet rear end can be increased. This preventspoor accommodation due to the leaving of the sheet rear end.

Further, the delivery roll pair 34 serves as the inverting roll forduplex printing. In the case of the duplex printing, the delivery rollpair 34 is inverted. In this case, as apparent from FIG. 2, since thefollower roll 102 is located at the position (right side in FIG. 2)nearer to the inverting path 95 than the pinch roll 100, when thedelivery roll pair 34 is employed as the inverting roll, the sheet rearend will not be left on the pinch roll 100.

An explanation will be given of the delivery roll pair 34 according tothe second embodiment of this invention.

FIG. 5 is a side view of the status in the vicinity of the delivery rollpair 34 according to the second embodiment of this invention. FIG. 6 isa side view of the delivery roll pair 34 shown in FIG. 5 when it isviewed from the side of the delivery tray 36.

As seen from FIGS. 5 and 6, the lower pinch roll 100 is circular insection at the zone in contact with the upper follower roll 100. Thepinch roll 100 is provided in a status wider than the follower roll 102.The pinch roll 100 is located on the side (left side in FIG. 5) nearerthe delivery tray 36 than the follower roll 102. At the center ofrotation of the pinch roll 100, a rotary axis 103 for rotation-drivingthe pinch roll 100 is provided. The pinch roll 100 includes a roll body104 and a plurality of convexes 106 formed circumferentially on bothends of the roll body 104 not in contact with the follower roll 102. Theconvexes 106 are provided for pushing the rear end of the sheet to bedelivered.

As regards the delivery roll pair 34, as in the delivery roll pairaccording to the first embodiment, one of these convexes 106 is arrangedto be more inside radially than the tangential line drawn from theradial tip 107 of another convex 106 upstream adjacent in the rotatingdirection to the convex 106 at issue to the outer periphery of the rollbody 104 of the pinch roll 100.

In the delivery roll pair 34, the distance L1 from the root 108 of eachof the convexes 106 formed on both sides of the roll body 104 to therotation center is made shorter than the outer diameter L2 of the rollbody 104.

Thus, even where the degree of the convex from the outer periphery ofthe roll body 104 is relatively small, a part of the sheet rear enddelivered by the delivery roll pair 34 is bent smoothly as shown in FIG.6 to enter between the adjacent convexes 106 in the rotating directionon the both ends in the axial direction of the pinch roll 100 so thatthe sheet rear end can be effectively pushed out. This obviates damageson the image on the sheet which is attributable to the convexes 106.

An explanation will be given of the delivery roll pair 34 according tothe third embodiment of this invention.

FIG. 7 is a side view of the status in the vicinity of the deliveryrolls 34 according to the third embodiment of this invention. FIG. 8 isa partial sectional view of the delivery roll pair 34 shown in FIG. 7when it is viewed from the side of the delivery tray 36.

The delivery roll pair 34 consists of a lower pinch roll 100 and anupper follower roll 102. The pinch roll 100 is circular in section atthe zone in contact with the follower roll 102. The pinch roll 100 isformed in a status wider than the follower roll 102 in an axialdirection, and located on the side (left side in FIG. 7) nearer thedelivery tray 36 than the follower roll 102. At the center of rotationof the pinch roll 100, a rotary axis 103 for rotation-driving the pinchroll 100 is provided.

The pinch roll 100 includesaroll body 104 and a plurality of convexes106 formed on both ends in the rotating direction of the roll body 104.The convexes 106 are provided for pushing the rear end of the sheet tobe delivered to shift inwardly in the radial direction through theirelastic deformation. Incidentally, these convexes 106 are formedintegrally to the roll body 104. The convexes 106 are formed in alightening shape or a shape with recesses on both ends in the axialdirection. As regards these convexes 106, when the sheet passes the nip35 while receiving the pressure not lower than a predetermined pressure,the convex 106 passing the same position as the nip 35 in the rotatingdirection shifts inwardly through its elastic deformation in the radialdirection of the pinch roll 100, i.e. sinks inwardly in the radialdirection of the pinch roll 100. On the other hand, when the sheetpasses the nip 35 while receiving the pressure lower than thepredetermined pressure, the convex 106 keeps its normal convex shape.Thus, for example, where a thick sheet, which receives the pressure notlower than the predetermined pressure, passes the nip 35, the convex 106passing the nip 35 shifts to the vicinity of the outer periphery of thepinch roll 100 and after having passed the nip 35, the convex 106returns to the original convex shape.

Thus, when the thick sheet which receives the pressure not lower thanthe predetermined pressure passes the nip 35, the thick sheet can beprevented from being damaged owing the convex shape. Further, byincreasing the degree of the convex, the performance of kicking out thesheet rear end can be improved.

An explanation will be given of the delivery roll pair 34 according tothe third embodiment of this invention.

FIG. 9 is a side view of the status in the vicinity of the delivery rollpair 34 according to the third embodiment of this invention. FIG. 10 isa partial sectional view of the delivery roll pair 34 shown in FIG. 9when it is viewed from the side of the delivery tray 36.

The delivery roll pair 34 consists of a lower pinch roll 100 and anupper follower roll 102. As seen from FIGS. 9 and 10, the pinch roll 100is circular in section at its area in contact with the follower roll102. The pinch roll 100 is formed in a status wider than the followerroll 102, and located on the side (left side in FIG. 9) nearer thedelivery tray 36 than the follower roll 102. At the center of rotationof the pinch roll 100, a rotary axis 103 for rotation-driving the pinchroll 100 is provided.

The pinch roll 100 includes a roll body 104 and a plurality of convexes106 for pushing the rear end of the sheet to be delivered. The convexes106 shift inwardly in the radial direction through their elasticdeformation. Incidentally, these convexes 106 are formed separately fromand made of a different material from the roll body 104.

The roll body 104 is made of any one oftetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA),polytetrafluoroethylene (PTFE) and tetrafluoroethylene-ethylenecopolymer (ETFE) which have good slidability and toner-releasibility. Onthe other hand, the convexes 106 is made of any one of polyacetal (POM),polypropylene (PP) and polycarbonate (PC) which have good elasticity andabrasion resistance.

Thus, even where the sheet passing distance from the fixing device 90 tothe delivery roll pair 34 is so short that the pinch roll 100 isrequired to have the releasiablity, the image quality and performance ofkicking out the sheet rear end can be improved.

Incidentally, all the convexes 106 are formed integrally. They aresupported by supporting segments 112 which extend from a cylindricalcoupling segment 110 to both left and right ends of the roll body 104.In order to mount the convexes 106 on the roll body 104, the supportingsegments 112 are deformed inwardly in the radial direction so that theyare inserted in the roll body 104, and after the convexes 106 on the endon the one side have passed the interior of the roll body 104, thesupporting segments 112 are restored to their original shape. Thus, theconvexes are mounted in the roll body 104.

Among these convexes 106, when the sheet passes the nip 35 whilereceiving the pressure not lower than a predetermined pressure, theconvex 106 passing the same position as the nip 35 in the rotatingdirection shifts through its elastic deformation to the vicinity of theouter periphery of the pinch roll 100. On the other hand, when the sheetpasses the nip 35 while receiving the pressure lower than thepredetermined pressure, the convex 106 keeps its normal convex shape.Thus, for example, where a thick sheet, which receives the pressure notlower than the predetermined pressure, passes the nip 35, the convex 106passing the nip 35 shifts to the vicinity of the outer periphery of thepinch roll 100 and after having passed the nip 35, the convex 106returns to the original convex shape, thus enabling the sheet rear endto be kicked out.

As described hitherto, this invention can be applied to a sheet deliverymechanism which when a sheet is kicked out by a convex attached to atleast one of a first roll and second roll of a sheet delivery roll pair,can prevent poor accommodation due to leaving of a sheet rear endwithout being hindered by another convex located on the downstream sideof the convex. Further, this invention can be applied to a sheetdelivery mechanism which when can deliver a thick sheet without damagingit.

1. A sheet delivery mechanism comprising: a first roll; a second roll;and a plurality of convexes formed circumferentially for pushing a rearend of a sheet to be delivered, wherein: a sheet delivery roll paircomprises the first roll and the second roll in contact with each other;at least one of the first roll and the second roll is provided with theplurality of convexes; and one of the plurality of convexes is arrangedto be more inside radially than the tangential line drawn from theradial tip of another convex upstream adjacent in the rotating directionto the one convex at issue to the outer periphery of the roll.
 2. Thesheet delivery mechanism according to claim 1, wherein a portion moreupstream than the radial tip of the one convex has a tapered shape witha gradient approximately equal to the tangential line drawn from theradial tip of another convex upstream adjacent in the rotating directionto the one convex to the outer periphery of the roll.
 3. The sheetdelivery mechanism according to claim 1, wherein the distance from theroot of the one convex to a rotation center is shorter than the outerdiameter of the roll.
 4. A sheet delivery mechanism comprising: a firstroll; a second roll; and a plurality of convexes formedcircumferentially for pushing the rear end of a sheet to be delivered toshift inwardly in the radial direction through their elasticdeformation, wherein a sheet delivery roll pair comprises the first rolland the second roll in contact with each other; at least one of thefirst roll and the second roll is provided with the plurality ofconvexes on its roll body; where a sheet which receives pressure notlower than a predetermined pressure when it passes a nip passes the nip,the convex passing the same position as the nip in the rotatingdirection shifts inwardly in the radial direction; and where the sheetwhich receives the pressure lower than the predetermined pressure whenit passes the nip passes the nip, the convex keeps its normal convexshape.
 5. The sheet delivery mechanism according to claim 4, wherein theroll body and the convexes are integrally formed of resin, and theconvexes are formed in a lightening shape or a shape with recesses onboth ends in the axial direction
 6. The sheet delivery mechanismaccording to claim 4, wherein the roll body and the convexes are formedseparately from each other and made of different materials from eachother
 7. The sheet delivery mechanism according to claim 6, wherein theroll body comprises at least one of atetrafluoroethylene-perfluoroalkylvinylether copolymer, apolytetrafluoroethylene and a tetrafluoroethylene-ethylene copolymer. 8.The sheet delivery mechanism according to claim 6, wherein the convexescomprises at least one of a polyacetal, a polypropylene and apolycarbonate.
 9. The sheet delivery mechanism according to claim 1,wherein the sheet delivery roll pair serves as an inverting roll forduplex printing.
 10. A fixing device comprising: a heating roll and apressurizing roll for fixing a developer image on a sheet; and a sheetdelivery mechanism including: a first roll; a second roll; and aplurality of convexes formed circumferentially for pushing a rear end ofthe sheet to be delivered, wherein: the heating roll and thepressurizing roll transport the sheet toward the sheet deliverymechanism; a sheet delivery roll pair comprises the first roll and thesecond roll in contact with each other; at least one of the first rolland the second roll is provided with the plurality of convexes; and oneof the plurality of convexes is arranged to be more inside radially thanthe tangential line drawn from the radial tip of another convex upstreamadjacent in the rotating direction to the one convex at issue to theouter periphery of the roll.
 11. An image forming apparatus comprising:a recording medium feeding unit for feeding a sheet; a fixing device forfixing a developer image on the sheet; and a transporting path that isarranged from the recording medium feeding unit to the fixing device,wherein the fixing device includes a sheet delivery mechanism including:a first roll; a second roll; and a plurality of convexes formedcircumferentially for pushing a rear end of the sheet to be delivered; asheet delivery roll pair comprises the first roll and the second roll incontact with each other; at least one of the first roll and the secondroll is provided with the plurality of convexes; and one of theplurality of convexes is arranged to be more inside radially than thetangential line drawn from the radial tip of another convex upstreamadjacent in the rotating direction to the one convex at issue to theouter periphery of the roll.